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Glycemic index (also glycaemic index, GI) is a ranking system for carbohydrates based on their effect on blood glucose levels. It compares carbohydrates gram for gram in individual foods, providing a numerical, evidence-based index of postprandial (post-meal) glycemia. The concept was invented by Dr. David J. Jenkins and colleagues in 1981 at the University of Toronto.

Carbohydrates that break down rapidly

The G.I. Diet
The G.I. Diet

during digestion have the highest glycemic indices. Carbohydrates that break down slowly, releasing glucose gradually into the blood stream, have a low glycemic index. A lower glycemic index suggests slower rates of digestion and absorption of the sugars and starches in the foods and may also indicate greater extraction from the liver and periphery of the products of carbohydrate digestion. A lower glycemic response is often thought to equate to a lower insulin demand, better long-term blood glucose control and a reduction in blood lipids. But in fact, some foods having a low glycemic index or having very little carbohydrate cause a high insulin response or raise blood lipids. The insulin index may therefore also be useful as it provides a direct measure of the insulin response to a food.

The glycemic index of a food is defined by the area under the two hour blood glucose response curve (AUC) following the ingestion of a fixed portion of carbohydrate (usually 50 g). The AUC of the test food is divided by the AUC of the standard (either glucose or white bread, giving two different definitions) and multiplied by 100.

The effect on blood glucose from a high versus low glycemic index carbohydrateThe average GI value is calculated from data collected in 10 human subjects. Both the standard and test food must contain an equal amount of available carbohydrate. The result gives a relative ranking for each tested food.

The current validated methods use glucose as the reference food, giving it a glycemic index value of 100 by definition. This has the advantages in that it is universal and it results in maximum GI values of approximately 100. White bread can also be used as a reference food, giving a different set of GI values (if white bread = 100, then glucose ≈ 140). For people whose staple carbohydrate source is white bread, this has the advantage of conveying directly whether replacement of the dietary staple .

The effect on blood glucose from a high versus low glycemic index carbohydrate
The effect on blood glucose from a high versus low glycemic index carbohydrate

with a different food would result in faster or slower blood glucose response. The disadvantages with this system are that the reference food is not well-defined, and the GI scale is culture dependent

Glycemic index of foods


GI values can be interpreted intuitively as percentages on an absolute scale and are commonly interpreted as follows:


GI range


Low GI

55 or less

most fruit and vegetables (but not potato), oats, buckwheat, whole barley, All-bran, Basmati rice

Medium GI

56 - 69

sucrose, Mars bar, croissant

High GI

70 or more

corn flakes, baked potato, Jasmine rice, white bread


 Classification GI range Examples
A low GI food will release glucose more slowly and steadily. A high GI food causes a more rapid rise in blood glucose levels and is suitable for energy recovery after endurance exercise or for a person with diabetes experiencing hypoglycemia.

The glycemic effect of foods depends on a number of factors such as the type of starch (amylose vs. amylopectin), physical entrapment of the starch molecules within the food, fat content of the food and increased acidity of the meal — adding vinegar for example, will lower the GI. The presence of fat or soluble dietary fibre can inhibit carbohydrate absorption, thus lowering the GI. Unrefined breads with higher amounts of fibre generally have a lower GI value than white breads, but, while adding butter or oil will lower the GI of bread, the GI ranking does not change. That is, with or without additions, there is still a higher blood glucose curve after white bread than after a low GI bread such as pumpernickel. Many brown breads, however, are treated with enzymes to soften the crust, which makes the starch more accessible. This raises the GI, with some brown breads even having GI values over 100.

The glycemic index can only be applied to foods with a reasonable carbohydrate content, as the test relies on subjects consuming enough of the test food to yield about 50 g of available carbohydrate. Many fruits and vegetables (but not potatoes) contain very little carbohydrate per serving, or have very low GI values. This also applies to carrots, which were originally and incorrectly reported as having a high GI. Alcoholic beverages have been reported to have low GI values, however it should be noted that beer has a moderate GI. Recent studies have shown that the consumption of an alcoholic drink prior to a meal reduces the GI of the meal by approximately 15%.

Disease prevention
Several lines of recent scientific evidence have shown that individuals who followed a low GI diet over many years were at a significantly lower risk for developing both type 2 diabetes and coronary heart disease than others, but the results obviously depend on the two diets being compared. High blood glucose levels or repeated glycemic "spikes" following a meal may promote these diseases by increasing oxidative damage to the vasculature and also by the direct increase in insulin levels. In the past, post-meal hyperglycemia has been a risk factor mainly associated with diabetes, however more recent evidence shows that postprandial hyperglycemia presents an increased risk for atherosclerosis in the non-diabetic population.

On the other hand there are regions, such as Peru and Asia, where people eat high-glycemic index foods such as potatoes and rice, but without a high level of obesity or diabetes. The high consumption of legumes in South America and fresh fruit and vegetables in Asia likely has a lowering glycemic effect in these individuals. The mixing of high and low GI carbohydrates produces moderate GI values.

The glycemic index is supported by leading international health organisations including the American Diabetes Association.


Weight control
Recent animal research provides compelling evidence that high GI carbohydrate is associated with increased risk of obesity. In human trials, it is typically difficult to separate the effects from GI and other potentially confounding factors such as fibre content, palatability, and compliance. In the study (Pawlak et al, 2004), male rats were split into high and low GI groups over 18 weeks while mean bodyweight was maintained. Rats fed the high GI diet were 71% fatter and had 8% less lean body mass than the low GI group. Postmeal glycemia and insulin levels were significantly higher and plasma triglycerides were three-fold greater in the high GI fed rats. Furthermore, pancreatic islet cells suffered "severely disorganised architecture and extensive fibrosis". The evidence in this study showed that continued consumption of high glycemic index carbohydrates would likely have led to the development of severe metabolic abnormalities.

Endurance athletes such as ultra-marathoners and iron man triathletes have become increasingly aware of the glycemic index of foods taken before and during training and competition. In the hours before a competition athletes may consume foods with a lower GI value so that energy is released more slowly. During competition, the athlete may try to balance the consumption of high and low glycemic index foods to maintain constant energy levels and avoid "energy spikes".

The glycemic index has been criticised for the following reasons:

  • The GI does not take into consideration other factors besides glycemic response, such as insulin response.
  • The GI of a food varies depending on the kind of food, its ripeness, the length of time it was stored, how it was cooked, its variety (potatoes from Australia, for example, have a much higher GI than potatoes from the United States), and how it was processed or manufactured.
  • The GI of a food varies from person to person and even in a single individual from day to day, depending on blood glucose levels, insulin resistance, and other factors.
  • The GI of a mixed meal can be difficult to predict.
  • The GI value is based on a portion that contains 50 grams of carbohydrate only.

This criticism can be addressed by taking the Glycemic load into account. This combined approach is, however, somewhat more complicated, and therefore harder to use in giving dietary advice.


About the G.I. Diet
All diets will let you lose weight so why do 95% of them fail? The answer is simply that people cannot sustain them because:

  • They feel hungry or deprived
  • Diets are too complex with measuring and counting of calories, grams, blocks, quotas etc.
  • They don’t feel healthy
  • Rick Gallop waged his own personal battle of the bulge, tried countless diets before discovering the Glycemic Index or G.I. and realized he had found the magic bullet that addressed all these issues head on.

Developed by Dr.David Jenkins, a professor of nutrition at the University of Toronto, the G.I. measures the speed at which foods are broken down by the body to form glucose, the body’s source of energy. High G.I. foods break down quickly and leave you looking for the next food fix. Low G.I. foods break down more slowly and leave you feeling fuller, longer. It is these low G.I. foods that form the core of the diet.

The G.I. Diet makes all the calculations for you by listing all foods in three traffic light colour categories: red light foods which you avoid if you want to lose weight; yellow light listings are foods that are to be used occasionally; and green light foods – eat as much as you like.

In short, The G.I. Diet will not let you go hungry or feel deprived. It is simplicity itself for as one Columnist put it ‘if you can follow a traffic light, you can follow this diet’. Finally it will not harm your health like many of today’s diets, but rather will actually reduce your risk from heart disease, stroke, type 2 diabetes and many cancers.

With recipes, snacks ideas, pull out shopping list, pantry and dining out guides, The G.I. Diet will be the way you will eat for the rest of your life.

External Links:
G I Diet Website




This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

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